1. Field of Invention
The present invention relates to a multi-speed automatic transmission employed in a vehicle, and particularly to a gear train thereof.
2. Description of Related Art
There have been demands for automatic transmissions mounted in vehicles to be multi-speed in order to ensure drivability and improve fuel consumption which is essential to conserve energy. In response to this demand there has arisen a need for further reduction of the number of gear change elements and engagement elements (hereinafter in this specification clutches and brakes will commonly be referred to as engagement elements) per speed of a gear train. Considering related art from this perspective, U.S. Pat. No. 5,226,862 discloses art which achieves six speeds by control according to five engagement elements by inputting two different rotations to a planetary gear set of four elements. This related art employs a structure wherein one element is a reaction force element which is engaged with a brake, which achieves a highest speed by increasing a rotation speed of an output element with respect to an input element.
In a planetary gear set of four elements, however, with the exception of a directly coupled speed, because one element is never involved with the transmission of power, rotation is not transferred with clutch engagement, nor is the element engaged by means of brake engagement because it is not a reaction force element. Therefore the element rotates freely because it is meshed with an element involved with the transmission of power. In particular with the related art, in the higher speeds of fifth speed and sixth speed, especially sixth speed, the highest speed which is used for the longest periods of time when normally driving a vehicle, the freely rotating element rotates at an extremely high speed along with the overdriven rotation of the output member. In this kind of state, resistance from the dragging of the disengaged engagement element connected to the freely rotating element, increases. This translates into a power transmission loss, thereby reducing transmission efficiency of the automatic transmission.
In view of this, it is a primary object of the present invention to improve transmission efficiency by reducing drag loss from an engagement element generated by an increase in the rotation speed of each element comprising the planetary gear set when each speed is achieved, and particularly in the highest speed. It is further an object of the present invention to improve transmission efficiency by making the highest speed the directly coupled speed and reducing gear transmission loss in an automatic transmission for a vehicle achieving multiple speeds by inputting two different speed ratios to the planetary gear set.
In order to achieve the foregoing objects, an automatic transmission for a vehicle according to the present invention comprises: a first input path having a fixed first speed ratio with respect to a rotation of an input shaft; a second input path having a fixed second speed ratio which is larger than the first speed ratio; a planetary gear set of four elements comprising a combination of a plurality of planetary gears, the four elements of the planetary gear set being a first element, a second element, a third element, and a fourth element following an order of elements on a speed diagram; a second clutch transmitting a rotation from the second input path to the first element; a first clutch transmitting the rotation from the second input path to the fourth element; a fourth clutch transmitting a rotation from the first input path to the first element; a third clutch transmitting the rotation from the first input path to the second element; a first brake engaging the fourth element; a second brake engaging the second element; and an output member coupled to the third element.
According to this aspect of the present invention, it is possible to prevent an increase in resistance from dragging of an engagement element connected to the freely rotating element because the element that would rotate freely does not rotate at an extremely high speed due to control of each element of the planetary gear set according to any engagement combination of the engagement elements. Moreover, control of the engagement elements while upshifting and downshifting is facilitated due to the fact that reverse rotation of the freely rotating element does not occur through any of the forward speeds.
According to another form of the foregoing aspect of the present invention, it is also effective to have, with respect to engagement of the first brake, a first speed achieved by engagement of the fourth clutch and a second speed achieved by engagement of the second clutch; with respect to engagement of the third clutch, a third speed achieved by engagement of the first brake, a fourth speed achieved by engagement of the second clutch, and a fifth speed achieved by engagement of the fourth clutch; with respect to the first clutch, a sixth speed achieved by engagement of the third clutch, a seventh speed achieved by engagement of the fourth clutch, and an eighth speed achieved by engagement of the second clutch.
According to this structure, it is possible to prevent an increase in resistance from dragging of an engagement element connected to a freely rotating element because extreme high speed rotation of an element that rotates freely when each forward speed is achieved is prevented, and particularly because, with the exception of third speed which is a low speed, free rotation that exceeds the input rotation does not occur. Control of the engagement elements while upshifting and downshifting is also facilitated due to the fact that reverse rotation of the freely rotating element does not occur through any of the forward speeds. Moreover, because the planetary gear set becomes directly coupled when the highest speed is achieved, it is possible to eliminate transmission loss of the planetary gear set in the highest speed, which is used for the longest periods of time during normal vehicle driving. Further, the combination of these characteristics improves the transmission efficiency of the automatic transmission.
According to yet another form of the foregoing aspect of the present invention, a structure may be employed which has an overdrive planetary gear for overdriving and outputting the rotation from the input shaft, wherein the input path from the overdrive planetary gear is the second input path, and the input path from the input shaft which does not pass through the overdrive planetary gear is the first input path.
According to this structure, rotation of the input path becomes directly coupled and overdriven rotation with respect to the input rotation. As a result, the speed ratio width from the lowest speed to the highest speed achieved based on these rotations extends in the direction of overdrive. This enables each gear ratio of multiple speeds to be set at a wide spread.
Further, according to still another form of the foregoing aspect of the present invention, it is also effective to have the planetary gear set comprise: a Ravigneaux type gear set in which the first element is a sun gear, the second element is a carrier supporting both a short pinion meshing with this sun gear and a long pinion meshing with this short pinion, the third element is a ring gear meshing with the long pinion, and the fourth element is another sun gear meshing with the long pinion.
According to this structure, it is possible to make a planetary gear set of four elements compact, in particular reducing the axial dimensions and thereby the axial length of the automatic transmission because the members connecting the elements of the planetary gear set are able to be eliminated.